The present invention relates to composite carrier particles comprising ferromagnetic fine particles and a phenol resin, and having a low bulk density and high electric resistance, and a process for producing such composite carrier particles. Such composite carrier particles have as high saturation magnetization as possible owing to the high content of ferromagnetic fine particles and are serviceable as magnetic carrier for electrophotography.
In electrophotography, a developing method is prevalently used in which an electrostatic latent image is formed by various means by using a photoconductive material such as selenium, OPC (Organic photoconductor), .alpha.-Si or the like as photoconductive material, and a toner electrically charged to an opposite polarity to the latent image is made to adhere to the latent image with electrostatic force by using, for instance, a magnetic brush development, thereby developing the latent image.
In the developing process, there are used carrier particles which are usually referred to simply as carrier, an appropriate quantity of positive or negative electricity is applied to the toner through frictional charging, and the charged toner is transferred to the developing zone near the surface of the photoconductive layer where the latent image is formed, through the medium of a magnet-incorporated development sleeve, by making use of magnetic force.
Recently, with increasing a tendency to speed-up, continuation and higher performance of copying machines, the a demand for the improvement of properties of carrier used in such copying machines.
The carrier used for the said purpose is required to have the following properties: low in bulk density, large in saturation magnetization and high in electric resistance.
When the bulk density of the carrier particles is high, there is required a large driving force for stirring in the developing apparatus, resulting in early mechanical wear, production of spent toner, deterioration of charging characteristics of the carrier itself and damage to the photoconductive layer. It is, therefore, keenly required that the carrier particles are low in bulk density.
Also, low saturation magnetization weakens the magnetic adhesive force of carrier to the development sleeve, thereby causing release cf the carrier particles from the development sleeve and their adhesion to the surface of the photoconductive layer. Thus, large saturation magnetization of the carrier particles has also been a strong requirement.
As for the electric resistance, it is required that the magnetic carrier have as high electric resistance as possible because of the necessity to control frictional chargeability of toner for forming a clear image.
Hitherto, iron-powder carrier, ferrite carrier and binder-type carrier (resin particles having fine magnetic particles dispersed therein) have been developed and practically used as a magnetic carrier.
The magnetic carrier particles having low bulk density, large saturation magnetization and high electric resistance are most keenly required at present, but there are no magnetic carrier particles yet available which can be amply satisfy these property requirements.
Regarding the iron particles carrier, there are available flaky particles, sponge-like particles or spherical particles, but since true specific gravity of these particles is 7 to 8, their bulk density is as high as 3 to 4 g/cm.sup.3 and their electric resistance is as low as 10.sup.2 to 10.sup.3 .OMEGA..cm, a large driving force is necessitated for stirring in the developing apparatus, which leads to early mechanical wear of the apparatus, resulting in production of spent toner, deterioration of charging characteristics of carrier itself and damage to photoconductive layer.
As a means for increasing electric resistance, it is practiced to treat the subject particles with an organic solvent containing a resin, thereby coating the surface of the iron-particles with the resin. According to this method, however, because of low throughput rate, the coating of the surface of the iron particles tends to become insufficient and non-uniform, and the effect of increasing the electric resistance is unsatisfactory. Therefore, the same treatment must be repeated several times. This causes complex and troublesome operations. Thus this method is disadvantageous industrially and economically. Further, oxide coating film of the surface of the iron particles is liable to peel off and also unstable as oxidation may take place and advance in certain environmental conditions. Thus, there tends to occur peeling and cracking of resin coating and the coated surface of the iron particles may be partly exposed, thereby causing disturbance of charging characteristics.
Ferrite particles carrier are spherical in shape, with their true specific gravity being about 4.5 to 5.5 and their bulk density being about 2 to 3 g/cm.sup.3. The ferrite particles carrier, therefore, can obviate the problem of weight which is the defect of the iron-powder carrier, but the ferrite particles carrier is still unable to adapt itself satisfactorily to high speed copying machines where the development sleeve or the magnet therein rotates at high speed, or high speed laser beam printers for general purpose computers.
Binder-type carrier is low in bulk density (less than 2 g/cm.sup.3), but as described in Japanese Patent Publication No. 59-24416 (1984), this binder-type carrier is produced by mixing and melting magnetic fine particles and a matrix resin, and then cooling and pulverizing the molten mixture. The produced particles, therefore, are low in magnetization, and accordingly they have the problem that their magnetic adhesive force to the development sleeve is weak, which tends to cause release of carrier particles from the development sleeve and adhesion to the photoconductive layer. Also, these carrier particles are irregular in shape and poor in fluidity, so that they are hard to stir and tend to cause non-uniformity in development, so that this binder-type carrier is unsatisfactory for its application to high-speed development where especially good fluidity of the developer is required.
It is also attempted to obtain a binder-type carrier having a curved particle-surface, especially a spherical binder-type carrier. It is possible, as described in Japanese Patent Application Laid-Open (KOKAI) No. 59 -1967 (1984), to obtain spherical particles by mixing a thermoplastic resin and ferromagnetic fine particles, pulverizing the resultant mixture and further subjecting it to hot-air treatment. But in this case, it is hardly possible to make the ferromagnetic fine particles content of not less than 80% by weight, and there are cases where it is impossible to secure magnetism necessary for preventing scattering of the carrier particles during high speed development, although designing of the developing apparatus is partly responsible therefor. In case of dispersing spinel ferrite particles such as magnetite particles for pigment having submicron in diameter into a thermoplastic resin by kneading, usually when the content of such spinel ferrite particles exceeds 80% by weight, there is noted a tendency that the hot-melt mixture increases in viscosity and decreases in fluidity, and as a result it is difficult to perform the kneading. Even if the kneading can be performed, it is hardly possible to make the pulverized particles spherical by a hot-air treatment because of the high viscosity of the melt.
In the production of a binder-type carrier, a thermoplastic resin is usually used as the matrix resin, but in this case, the produced magnetic carrier particles are weak in strength and may be split into finer particles, which may become a cause of fogging of the developed image. In Japanese Patent Application Laid-Open (KOKAI) No. 58-136052 (1983) the use of a thermosetting resin in place of thermoplastic resin for improving strength of magnetic particles carrier is proposed. But in this case, it is also hardly possible to make the content of the magnetic particles not lower than 80% by weight. In this Japanese KOKAI, as a process for producing binder-type carrier by using a thermosetting resin, a process in which a thermosetting resin and magnetic fine particles are mixed, the resultant mixture is melted and then heat-cured by adding a curing agent, and the resulting cured product is pulverized and classified is disclosed. According to this method, however, it is impossible to obtain spherical particles by a hot-air treatment since the resin is thermoset, and the classified-out unnecessary particles can not be recycled unlike in the case of using a thermoplastic resin, so that industrial application of this method is difficult in terms of cost. As another process for producing binder-type carrier by using a thermosetting resin, the said Japanese KOKAI also discloses a method in which a thermosetting resin is dissolved in a solvent such as toluene, then magnetic fine particles are dispersed therein, and the resultant dispersion is sprayed for granulation and then dried to evaporate way the solvent. The resulting granulated particles are further heat-cured and classified to form the desired carrier particles. According to this method, it is easy to form spherical particles, but since the process involves evaporation of a large amount of solvent, voids are apt to form in the granulated particles, thereby impairing their strength. Also, an apparatus for recovering a large amount of solvent is necessitated, and the classified-out particles with undesired sizes can not be recycled as in the case of the said pulverization method. This method, therefore, is unsuited for practical application. As described above, a variety of carrier particles and processes for producing the carrier particles have been proposed, and some of them have been put to practical use. However, for use in digital copying machines having the latest digital techniques applied to electrophotography, laser beam printers, plain paper facsimiles and other high-technique office machines, there are required the carrier particles having higher performance, that is, the particles which can enable even higher speed operations, higher image quality, higher fineness, and formation of clear color images. Such particles are required to be low in bulk density, to have a curved surface configuration and to be high in content of the ferromagnetic fine particles.
As a result of extensive studies on the process for obtaining the carrier particles having a curved surface configuration, low in bulk density, high saturation magnetization and high electric resistance, it has been found that composite carrier particles comprising more than 80% by weight to not more than 99% by weight of ferromagnetic fine particles and a phenol resin, obtained by reacting phenols and aldehydes in the presence of the ferromagnetic fine particles and a suspension stabilizer in an aqueous medium by using a basic catalyst, have a number-average particle diameter of 10 to 1,000 .mu.m, a bulk density of not more than 2.0 g/cm.sup.3 and a curved surface configuration, and are possessed of high saturation magnetization and high electric resistance. The present invention has been achieved on the basis of this finding.